Classic Truck Suspension And Steering Linkage Basics - Night School

Through the 1950s, all domestic trucks featured a ladder frame with leaf springs front and rear. These semi-elliptic springs arch below the framerails. The steering knuckles pivot on kingpins at each end of a solid axle beam. A truck's load capacity reflected its frame strength, axle beam size, steering gear, linkage stamina, spring load rates, and brake capacity.

Use of a beam front axle with a torque tube or Hotchkiss rear axle was common. Ford pickups through 1941 had a transverse leaf spring front and rear with a torque tube rear axle. (Radius rods in wishbone form lend support to the front axle beam and torque tube rear axle.) GM used semi-elliptic leaf springs front and rear with a closed, torque tube rear axle on 1/2-ton trucks through first-series 1955 models. When second-series 1955 GM models rolled from the assembly lines, the solid Hotchkiss rear axle with an open driveshaft became a mainstay for all American trucks.

The use of torsion bars was not widely accepted. At the time, only Chrysler's cars used torsion bars with IFS. By 1963, GM light trucks adopted double A-arm front suspension with coil springs. Eventually, rear link-and-coil suspension would yield to a traditional leaf-spring layout. A front stabilizer bar, spindles with ball joints, a rear track or panhard bar and other passenger car features were present in GM's 1960 truck suspension-in a far more rugged form, of course!

2/19In the pre- and postwar era, domestic truck builders used a ladder frame with forged beam front axle. A semi-elliptic leaf-spring stack perches at each side (as shown here). From the Model T era through 1941, Ford cars and light trucks use a beam front axle with a transversely mounted leaf spring.

Ford, Dodge, I-H, and Studebaker light trucks maintained their one-piece beam front axles for several more years. Ford broke ground in 1965 with twin I-beam front suspension. The twin half-axle system closely maintains the stamina and proven worth of beam axles.

These half-axles are offset, and their pivot points overlap. Radius arms support the beams fore-and-aft. Coil springs near each axle end replace conventional leaf springs for longer travel and improved ride quality. Maintaining conventional kingpins and spindles, twin I-beam suspension qualifies as independent front suspension but falls far short of the ride quality and efficient suspension geometry found in GM's unequal length A-arm truck suspension.

Suspension Wear
When inspecting a truck's suspension system, check out all pivots and moving parts. On vintage models, the axle beam must be true, not bent or damaged. Make sure the kingpins and spindles fit properly before checking the beam for straightness. Loose spindle bushings will distort the caster and camber readings. A bent or worn kingpin will also distort these measurements. Camber and caster angles are the best indicator of a beam axle's condition. (Learn more about caster and camber in next month's lesson.)

3/19The steering points of a beam front axle are the spindle and kingpin. This proven design serves well and is still a mainstay on larger truck chassis. Studebaker ended its history of light truck production with a traditional beam front axle and semi-elliptic springs. Longevity of kingpins is evident on this vintage Studebaker pickup axle.

On kingpin suspension with leaf springs, the spring bushings and shackles are a common wear point. This also applies to rear leaf springs. Later trucks use rubberized spring bushings, which wear and deteriorate over time. Early chassis use bronze bushings at the leaf-spring anchor eyes and the shackle ends. Play, looseness, or deterioration is a sign of failure.

Check for kingpin play by jacking the truck up and placing the beam axle on safety stands. Grip the wheel and tire at 6 and 12 o'clock. Rock the wheel while looking inside at the spindle and kingpin area. (This usually takes an assistant.) Movement between the spindle and kingpin is an indication of play. Anything more than the slightest movement is too much. New pins and new, ream-fitted bushings should fit together with no perceptible play.

Ball joints on an IFS are similar to passenger car parts. The ball joints at the top and bottom of each steering knuckle are a distinct wear point. With a floor jack beneath the lower A-arm, vehicle weight will compress the spring and unload pressure from the spindle's ball joints. You can move the tire and wheel at the 6 to 12 o'clock position and feel for loose ball joints. Ball joints cannot be tested under coil spring load or when vehicle weight is on the tire.

10/19IFS with double A-arms is common on GM two-wheel drive trucks since 1960. The steering spindle/knuckle assembly pivots on upper and lower ball joints. These ball-stud joints wear over time. If loose, the front end may shimmy, much like a traditional "kingpin shimmy" caused by loose kingpin bushings.

Steering Linkage Wear Points
Although steering linkage design varies between beam axle and IFS models, the steering linkage joints are of similar design. Steering linkage must pivot and allow for suspension movement. For this reason, tie-rod ends are of ball stud and socket design. Some pitman and idler arms use a ball stud and socket as well.

On older steering linkage, the joints at the drag link ends, or even the tie-rods, will have spring loaded ball cups. An adjustable plug sets a precise load on the cup tension spring. The steering arms or pitman arm have a ball head, which the cup halves capture-these joints, like all others, require periodic lubrication.

11/19

When checking for wear, a dry or semi-frozen joint may not show play. Sometimes, it is necessary to grease a linkage joint to force out dirt and debris before testing for play. A seemingly tight ball stud might become excessively loose.

IFS suspension has the most steering linkage joints. The centerlink or relay rod attaches to the steering gear's pitman arm. The idler arm supports the opposite end of the center link. On double A-arm suspension, the idler arm pivots from the right side framerail. The idler arm is a common wear item. Ford's twin I-beam steering linkage has a simpler "Y" design that uses fewer joints and does not require an idler arm.

When a beam axle model has the pitman arm moving laterally, a drag link or "short tie rod" connects the pitman arm to the steering arm at the right side spindle. A long, single tie rod connects the right and left steering arms. Another approach is the fore-and-aft moving pitman arm that moves a drag link. The drag link attaches to the left steering arm at the knuckle. A long tie rod connects the left and right steering arms.

12/19Sway bar bushings and rubber suspension stops also wear and deteriorate. There are OEM-type replacement bushings available. Some use aftermarket urethane bushings for more precise handling. A sway bar makes the chassis squat on hard turns, a means of stiffening suspension while lowering the center of gravity for safer cornering.

Steering linkage adjustments include toe-in and centering the steering gear. Next month, our discussion will cover the fundamentals of wheel alignment. Steering linkage and A-arm adjustments are central to frontend alignment. Stay tuned!

What Did You Learn This Month?
Night School would not be complete without a quiz! Don't worry about your test-taking skills or grades. This is an open-magazine, true or false test. Clues can be found within the Night School text, photos and captions. Have a good month!